A pipe implements PipeTransform and operates on arguments passed to a controller handler. Its two uses are transformation (converting input to a different type, e.g. ParseIntPipe converts a string param to a number) and validation (throwing BadRequestException if input fails rules, e.g. ValidationPipe with class-validator).
ParseIntPipe — converts string to integer, throws 400 if not parseable.
ParseFloatPipe — converts string to float.
ParseBoolPipe — converts 'true'/'false' strings to boolean.
ParseUUIDPipe — validates and passes through a UUID string.
ParseArrayPipe — parses comma-separated string into an array.
ValidationPipe — validates DTOs using class-validator decorators.
DefaultValuePipe — provides a fallback value if the input is undefined.